Field of the Invention
The invention pertains to processes for washing partly-hydrolyzed cellulose to remove acid to low levels.
Background of the Invention
The following references describe the process steps which have been employed to produce partly-hydrolyzed cellulose in the prior art literature and are discussed below, such as (1) U.S. Pat. No. 5,629,055 to Revol et al.; (2) Jean-François Revol, Louis Godbout, Xue-Min Dong, Derek G. Gray, Henri Chanzy, and Georg Maret, “Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation,” Liquid Crystals, (1994) Vol. 16, No. 1: 127; and (3) Xue Min Dong, Tsunehisa Kimura, Jean-Francois Revol, and Derek G. Gray, “Effects of Ionic Strength on the Isotropic-Chiral Nematic Phase Transition of Suspensions of Cellulose Crystallites,” Langmuir, (1996) Vol. 12: 2076.
Partly-hydrolyzed cellulose (also referred to in the literature as cellulose crystallites or nanocrystalline cellulose) is most commonly produced by the controlled acid hydrolysis of cellulose using sulfuric acid. The cellulose may be from various sources, including wood pulp and cotton. The less dense amorphous regions along the cellulose microfibril are more susceptible to acid attack during hydrolysis and cleave to give the partly-hydrolyzed cellulose. Acids other than sulfuric, such as hydrochloric acid, phosphoric acid, etc., or mixtures of acids, may be used. For simplicity of explanation, the following discussion focuses on the use of sulfuric acid and the removal of sulfate.
At high ionic strengths, such as that caused by the residual sulfuric acid from hydrolysis, the individual nano-particles of crystalline cellulose tend to agglomerate together into larger particles which settle under gravity, or in a centrifuge. To obtain the desired discrete nano-particles, the ionic strength of the solution must be reduced to very low levels. To achieve this desired purification, one or more steps of dilution followed by centrifugation are typically employed to separate the bulk of the sulfuric acid (about 90%) from the partly-hydrolyzed cellulose; however, as the acid concentration and associated ionic strength decrease, the partly-hydrolyzed cellulose particles deagglomerate into smaller aggregates to a point where they are too small to be collected efficiently in a centrifuge and alternative methods must be employed to further reduce the acid concentration.
To achieve the desired final purity of the partly-hydrolyzed cellulose suspension, a very fine filter which retains the partly-hydrolyzed cellulose particles combined with continuous or step-wise water washing is typically employed. This washing step is often referred to in the literature as dialysis or diafiltration.
Although a significant portion of the starting acid has usually been removed previously by centrifugation, purity targets for partly-hydrolyzed cellulose are such that acid concentrations must be further reduced by several orders of magnitude. Even with a highly efficient dialysis system, very large filtration surface areas and flow rates are required relative to the quantity of partly-hydrolyzed cellulose produced, leading to high capital and operating costs, water usage and layout requirements, especially at larger scales.
While generally improving overall viability, centrifugation is not strictly required as an initial purification step. The desired final purity can be achieved using dialysis alone; however, this increases dialysis washing requirements and acid recovery costs significantly.
The prior art purification process described above also removes sugars produced in hydrolysis as well as other soluble impurities; however, these are typically present in lower quantities than the sulfuric acid such that their removal is not normally the limiting factor in the purification of partly-hydrolyzed cellulose.
As described in the prior art indicated above, partly-hydrolyzed cellulose particles produced using sulfuric acid generally contain sulfate groups which are weakly acidic. Once the ionic strength has been reduced to sufficiently low levels, the partly-hydrolyzed cellulose suspension is typically dispersed using sonication or other high shear operation. In addition the partly-hydrolyzed cellulose suspension is usually neutralized with NaOH, or other base to improve the partly-hydrolyzed cellulose thermal stability and redispersability (if dried). Bases containing divalent cations, such as Ca++ and Ba++ are known to interfere with ordered phase formation such as the formation of iridescent films, etc. and so their presence in the final product is undesirable for many applications: “Cellulose Nanocrystal Suspensions,” Dr. Gray, D., Dept. of Chemistry, McGill University, 10th International Conference on Wood and Other Biofibre Plastic Composites, Madison, Wis., May 13, 2009.